portmaster VS opensnitch

There's a very good relatively new open-source GUI firewall app like this called Portmaster:

https://safing.io/

It's available for Windows and Linux

You can get your own firewall app. Portmaster by safing.io lets you open and close what ever ports you want on your windows system

amazing work OP! what's the difference between zen and portmaster? :) i would like to compare options to see which one fits my needs the best way

Portmaster itself is open source (https://github.com/safing/portmaster)

Then install https://safing.io/ (Portmaster); it's free.

I was working on an old game; trying to understand its Network protocol, and how the binary behavior when something on the connections goes wrong, and something comes to my mind: "What if I could block the traffic to this server only for this process?" what brings me to OpenSnitch.

Have you seen OpenSnitch? https://github.com/evilsocket/opensnitch

It's a Linux project that's replication what Little Snitch does on macOS - it doesn't decrypt TLS secured data but it does show and allow blocking of network connections (even if it can't see exactly what's going on inside this connections).

Combining eCapture features with OpenSnitch would be awesome. It'd be great if as well as tracking all network connection, you could flag connections sending specific data (like your name, email address, or phone number) to unexpected servers.

opensnitch has existed for a while now. I've never used it, so I can't comment on how well it works.

https://github.com/evilsocket/opensnitch

If you prefer a GUI try https://github.com/evilsocket/opensnitch

Close to zero most probably. If you want something different, to block/monitor what applications access the internet, block ads, etc, try https://github.com/evilsocket/opensnitch